Imaging apparatuses of a compact thin type have become so as to be installed in mobile terminal devices which are electronic devices of a compact thin type, such as a mobile telephone and a PDA (Personal Digital Assistance). With this, it has become possible to transmit mutually not only voice information, but also image information to distant places.
As image sensors used for these imaging devices, solid state imaging elements, such as CCD type image sensors and CMOS type image sensors, have been used. In recent years, a trend to make the density of pixels of an imaging element higher has been advanced, and making each of an image resolution and a performance higher has been attempted. Further, as a lens to form an image of a photographic object on these imaging elements, a lens made of resin suitable for mass production has been used in order to lower the cost more. Furthermore, since the lens made of resin has a good workability, it has responded to a request for higher performance by being shaped in an aspheric surface configuration.
As such an imaging lens used for an imaging apparatus incorporated in a mobile terminal device, a type with a three plastic lens structure and an optical system with a three lens structure composed of a single glass lens and two plastic lenses have been generally well known. However, in a situation that a request for these imaging lenses to be made more compact and a request for mobile terminal devices to be produced in a mass production process become severer, it becomes more difficult to respond to both of the requests simultaneously.
In order to overcome these problems, a technique to produce lens modules in the following mass production process has been proposed. Lens elements are mass produced simultaneously on a several inch glass base plate being a parallel flat plate by a replica method, the glass base plate (lens wafer) on which the large number of these lens elements are formed is combined with a sensor wafer, and then the lens elements are separated from each other into respective lens modules. The lenses produced in the above mass production process are called wafer scale lenses, and the lens modules are called wafer scale lens modules.
Further, in addition to the technique to produce lens modules in the mass production process, as a method of mounting the large numbers of lens modules on a base plate at low cost, in recent years, the following method has been proposed. That is, together with IC (Integrated Circuit) chips and other electronic components, lens modules are placed on a base plate on which solder has been coated previously by potting, and then the lens modules are subjected to a reflow process (heating process) while being placed on the base plate. Accordingly, by melting the solder, the lens modules and the electronic components are mounted simultaneously on the base plate. Therefore, the imaging lenses are requested to be excellent in heat resistance so as to be durable for the reflow process.